Nickel hydroxide is used as a cathode material for alkaline secondary batteries such as nickel/cadmium or nickel/metal hydride batteries. An electrochemical variable crucial for industrial use is capacity per unit mass. It is a function of the nickel content of the nickel hydroxide and of nickel utilization. The interrelationship between capacity per unit mass Q in mAh/g, the nickel content in wt. % of the cathode material and nickel utilization in % EES (one-electron step) is obtained as follows:   Q  =                              wt          .                                          ⁢          %                ⁢                                  ⁢                  Ni          ·          %                ⁢                                  ⁢                  EES          ·          F                            58.69        ·        3600        ·        10              [          mAh      ⁢              /            ⁢      g        ]  wherein    F=Faraday constant=96487A·s/mol     58.69=Molar mass of nickel, g/mol    3600=factor for converting seconds into hours    10=factor for converting % into ratio figures and mA into A.
This provides a theoretical capacity per unit mass at 100% EES of 289 mAh/g. Nickel hydroxides suitable for industrial use, however, comprise modification metal contents of between 4 and 6% and 1 to 2% water relative to the total metal content, such that, in the case of 100% nickel utilization, these comprise a capacity per unit mass of only 252 to 266 mAh/g. The 100% nickel utilization of this nickel hydroxide is associated with the 1-electron step (and 1-proton step) per nickel atom which occurs on conversion of β-Ni(II)(OH)2 into β-Ni(III)OOH during battery charging.
Overcharging of the battery results in further deprotonation and oxidation of some of the nickel into Ni(VI), i.e., in the formation of γ-NiOOH. For charge compensation, water molecules and alkali ions are incorporated in intermediate layers. Formation of γ-NiOOH is consequently associated with an expansion of the nickel layer spacing in the lattice. Discharging of the γ-NiOOH proceeds by release of alkali ions into the electrolyte, wherein α-Ni(OH)2 forms. The α-γ transition is theoretically provided with a 1.67-electron step, e.g. 167% EES. However, this 1.67-electron step cannot be used industrially, since the α-Ni(OH)2 in the alkaline electrolyte is unstable and is converted by ageing into β-Ni(OH)2, with reformation of the volume increase which occurred during the β-NiOOH-γ-NiOOH transition.
By introducing trivalent metals into nickel hydroxide it is possible to stabilize the a structure (pyroaurite). However, in the case of a pyroaurite structure, anions and water molecules are incorporated between the nickel hydroxide layers to ensure charge neutrality, such that the lattice is expanded. Although it is possible to achieve a stable 1.5-electron step per nickel atom in the case of such nickel hydroxides with pyroaurite structure modified by trivalent ions, the storage density is reduced on the one hand by the electrochemically inactive trivalent ions and on the other hand by the incorporated anions and water molecules.